NFC tag devices are employed in Radio Frequency Identification (RFID) systems that allow communication between an NFC tag device and a corresponding NFC reader device using inductive coupling. During this communication the reader mainly sends commands to the tag. Subsequently, the tag answers by transmitting data, e.g., identification information to the reader. Usually, tag and reader devices operate according to industry standards. ISO/IEC 14443 is an example for such industry standard which is also underlying the present application.
ISO/IEC 14443 defines contactless chip cards with an integrated NFC tag device in proximity coupling applications. Readers operating according to the standard use signals with a frequency of 13.56 MHz, called carrier frequency, to transfer data to a tag device. The tag device, which is also called a transponder, transmits data to the reader using signals with a frequency which is an integer divider of the carrier frequency, e.g., 13.56 MHz/16 and is named the subcarrier frequency. According to ISO/IEC 14443, the transponder is powered by the field of the reader and applies a load modulation to the reader's signal when transmitting data.
The operating range of RFID systems is limited by a coupling factor between an antenna of the reader and an antenna of the tag. The coupling factor represents a measure for the strength of inductive coupling between reader and tag and basically is a function of distance and angle between reader and tag antennae, as well as, a function of mechanical characteristics or geometry of the antennae. In order to increase the operating range or to decrease the antenna size, concepts have been developed which provide chip cards with a power source, e.g., a battery which enables these chip cards to actively generate a load modulated signal with the subcarrier frequency, thereby emulating the standard ISO/IEC 14443 passive load modulation. This process is called active load modulation.
The actively generated signal is synchronized to the signal of the reader. The active modulation allows using miniature antennas while maintaining transaction distance equal to or longer than the legacy, i.e., passive contactless cards. Having the option to use a miniature antenna is advantageous in mobile phones or wearable devices where space is the most critical constraint. The cost of the antenna is also reduced. A long operating range is likewise important for a good user experience and hence for the adoption of the contactless technology by the mass market in applications such as mobile phones.
A tag or tag device that employs active load modulation is called an active tag. The signal generated by the reader is also called a reference signal, a carrier signal or a reader signal. The signal generated by the tag during active load modulation is also named active load modulation signal.